Molecular basis of Cyperus difformis cross-resistance to ALS-inhibiting herbicides

Multiple herbicide resistance in a Cyperus difformis population in rice field from China

Herbicide resistance is rapidly emerging in Cyperus difformis in rice fields across China. The response of a C. difformis population GX-35 was tested against five acetolactate synthase (ALS)-inhibiting herbicides, auxin herbicide MCPA and photosynthesis II (PSII)-inhibitor bentazone. Population GX-35 evolved multiple resistance to ALS-inhibiting herbicides (penoxsulam, bispyribac‑sodium, pyrazosulfuron-ethyl, halosulfuron-methly and imazapic) and auxin herbicide MCPA, with resistance levels of 140-, 1253-, 578-, 18-, 13-, and 21-fold, respectively, compared to the susceptible population. In this population, ALS gene expression was similar to that of the susceptible population. However, an Asp376Glu mutation in ALS gene was observed, leading to reduced inhibition of in-vitro ALS activities by five ALS-inhibiting herbicides. Furthermore, CYP71D8, CYP77A3, CYP78A5 and three ABC transporter genes (cluster-14412.23067, cluster-14412.25321, and cluster-14412.24716) over-expressed in absence of penoxsulam. On the other hand, an UGT73C1 and an ABC transporter (cluster-14412.25038) were induced by penoxsulam. Additionally, both over-expression and induction were observed for CYP74, CYP71A1, UGT88A1 and an ABC transporter (cluster-14412.21723). The GX-35 population has indeed evolved multiple herbicide resistance in China. Therefore, a diverse range of weed control tactics should be implemented in rice field.

Cytochrome P450s-Involved Enhanced Metabolism Contributes to the High Level of Nicosulfuron Resistance in Digitaria sanguinalis from China

Large crabgrass (Digitaria sanguinalis (L.) Scop.) is one of the major malignant grass weeds in Chinese maize (Zea mays L.) fields, and it has recently developed resistance to the acetolactate synthase (ALS)-inhibiting herbicide nicosulfuron. This study focused on a suspected nicosulfuron-resistant (R) population (LJ-01) of D. sanguinalis, collected from Lujiang County in Anhui Province, China, to explore the resistance level and potential resistance mechanism. Whole-plant dose-response testing confirmed that the LJ-01 population evolved a high level of resistance to nicosulfuron (11.5-fold) compared to the susceptible (S) population, DY-02. The ALS gene sequencing and relative expression assay of the R plants indicated that target gene mutation and overexpression were not responsible for the resistance phenotype. However, pretreatment with malathion, a known cytochrome P450 monooxygenase (P450) inhibitor, alleviated the resistance of the R population to nicosulfuron by approximately 36%. High-performance liquid chromatography (HPLC) analysis revealed that the R plants metabolized nicosulfuron faster than the S plants. Moreover, cross-resistance testing suggested that the R population exhibited low levels of resistance to thifensulfuron-methyl and pyrazosulfuron-ethyl, but it remained susceptible to rimsulfuron. Multiple resistance patterns showed that the R population evolved low resistance to the photosystem inhibitors bromoxynil octanoate and atrazine and sensitivity to the acetyl-CoA carboxylase (ACCase) inhibitor cyhalofop-butyl and the 4-hydroxyphenylpyruvate dioxygenase (HPPD) inhibitors tembotrione, mesotrione, and topramezone. This study reports, for the first time, the simultaneous resistance to ALS and different photosystem inhibitors in D. sanguinalis. The nicosulfuron resistance observed in the R population could primarily be attributed to an enhanced metabolism involving P450 enzymes.

Target and nontarget mechanisms of AHAS inhibitor cross-resistance patterns in Cyperus difformis

Weed resistance to acetohydroxyacid synthase (AHAS) inhibiting herbicides has been a critical issue for rice growers worldwide since the early 1990's. In California, resistance to bensulfuron-methyl was first detected in Cyperus difformis in 1993. Since then, populations of most major weeds of rice in California have been reported to show resistance to at least one AHAS inhibitor. We sought to describe the magnitude and mechanisms of AHAS inhibitor cross-resistance in California populations of C. difformis. Sixty-two populations were collected and screened for cross-resistance to bensulfuron-methyl (BEN), halosulfuron-methyl (HAL), bispyribac‑sodium (BIS), and penoxsulam (PEN), revealing six major patterns of cross-resistance. Representative C. difformis populations from each cross-resistance pattern were then subjected to dose-response, cytochrome P450 inhibition, AHAS gene sequencing, and metabolic studies with the same herbicides as in the screening. Dose-response confirmed the detected resistances in the representative populations, and suggested that the majority of observed resistance was dose-dependent. Cytochrome P450 inhibition via malathion revealed evidence of increased metabolic activity in resistant populations to BEN, BIS, and PEN. AHAS gene sequencing revealed amino acid substitutions in five of six populations: R3 (Pro197-Ser), R4 (Pro97-His), R10 (Asp376), R41 (Ala122-Asn), and R18 (Trp574-Leu). Metabolic studies confirmed evidence of increased activity of cytochrome P450s in all populations. Metabolic BEN and HAL analysis did not yield similar results to malathion inhibition, suggesting different P450's or other pathways. Taken together, the results of the studies confirm the complexity of AHAS inhibitor cross-resistance in C. difformis, and the presence of both target-site and metabolic resistance in most of the representative populations underscores the importance of proper herbicide selection, rotation, and scouting in fields.

Mutation at the 197 site and P450-mediated metabolic resistance are involved in bensulfuron-methyl resistance in Sagittaria trifolia

Sagittaria trifolia control is threatened by the emergence of resistance to acetolactate synthase (ALS)-inhibiting herbicides. Hence, we systematically uncovered the molecular mechanism of resistance to the main herbicide (bensulfuron-methyl) in Liaoning Province from target-site and non-target-site resistance perspectives. The suspected resistant population (TR-1) exhibited high-level resistance. A new amino acid substitution (Pro-197-Ala) in resistant Sagittaria trifolia for ALS was detected, and the molecular docking results showed that the spatial structure of ALS changed significantly after the substitution, manifested by an increase in the number of contacted amino acid residues and the disappearance of hydrogen bonds. Dose-response test of transgenic Arabidopsis thaliana further demonstrated that the Pro-197-Ala substitution conferred bensulfuron-methyl resistance. The assays found that the sensitivity of the ALS enzyme in TR-1 to this herbicide was decreased in vitro; and this population had developed resistance to other types of ALS-inhibiting herbicides. Furthermore, the resistance of TR-1 to bensulfuron-methyl was significantly alleviated after co-treatment with a P450-inhibitor (malathion). TR-1 metabolized bensulfuron-methyl significantly faster than sensitive population (TS-1) did, but this gap was narrowed after malathion treatment. Overall, the resistance of Sagittaria trifolia to bensulfuron-methyl was derived from the mutation of the target-site gene and the enhancement of the P450s-mediated detoxification metabolism.

Light restriction associated with halosulfuron methyl application efficiently reduces the number and mass of tubers of Cyperus rotundus L

This study evaluated the effect of light availability in the culture environment and the application of a post emergence herbicide, halosulfuron methyl, on the management of Cyperus rotundus. The experiment was arranged in a 2 × 6 factorial design; the first factor was two levels of light availability: photosynthetically active radiation at 1180.4 and 411.6 µmols m^-2 s^-1, and the second factor was halosulfuron methyl doses from 28.13 to 140.62 g ha^-1. Photosynthetic efficiency, biomass allocation, accumulation of starch in tubers, and percentage control of C. rotundus were evaluated from 7 to 28 days after herbicide application. Doses greater than 70.30 g ha^-1 of halosulfuron methyl were efficient to control C. rotundus, regardless of light availability. However, C. rotundus was managed faster under full sunlight than under shading. The efficiency of the photosystem, starch accumulation, and biomass formation decreased with increasing doses of halosulfuron methyl. In a shaded environment, a dose of 28.13 g ha^-1 was sufficient to reduce 96.74% of the dry mass and 91.33% of the number of C. rotundus tubers. The decrease in light intensity associated with the use of halosulfuron methyl represents a promising practice for the control of C. rotundus.

Molecular basis of cross-resistance to acetohydroxy acid synthase-inhibiting herbicides in Sagittaria trifolia L

Acetohydroxy acid synthase (AHAS)-inhibiting herbicides are one of the most commonly used herbicides for controlling the growth of Sagittaria trifolia L. in paddy fields in Northeastern China. In this study, we collected five suspected resistant populations of S. trifolia (R1-R5) from three different provinces of Northeastern China. The results of whole-plant bioassays revealed that those populations showed high level of resistance to bensulfuron-methyl with resistance index (GR₅₀ R/S) ranging from 39.90 to 88.50. The results of AHAS-activity assays were consistent with the results of the whole-plant bioassays. The AHAS gene analysis showed that R2 and R3 populations contained Pro-197-Leu mutations that were highly resistant to penoxsulam; R1 and R4 populations contained Pro-197-Ser mutations that were highly resistant to bispyribac‑sodium; R5 population contained Trp-574-Leu mutation that showed high resistance to IMI, PT, PTB and SU herbicides. The AHAS with resistance mutations showed less sensitivity to feedback inhibition by BCAAs and R genotypes had increased free BCAAs.

Variation in mutations providing resistance to acetohydroxyacid synthase inhibitors in Cyperus difformis in China

Cyperus difformis has evolved resistance to pyrazosulfuron-ethyl and other acetohydroxyacid synthase (AHAS) inhibitors in paddy fields in China. To understand the distribution of resistance and the mutations involved, 38 populations collected were from 7 provinces and compared. Application of pyrazosulfuron-ethyl at 30 g a.i. ha^-1 identified 16 populations that survived, demonstrating resistance to this herbicide. Two exons of 498 and 1428 bp in length and a 1228-1233-bp intron of AHAS were cloned by genome walking, and three pairs of primers were designed to amplify eight conserved regions in this gene. In the 16 resistant (R) populations, five different types of mutations in the conserved region of the AHAS gene were identified: Pro-197-Ser, Pro-197-Arg, Pro-197-Leu, Asp-376-Glu, and Trp-574-Leu. Three R populations, YX15-22, YX12-10 and YX15-38, were chosen for in vitro AHAS activity assays, and the results showed that AHAS from YX15-22 carrying the Pro-197 mutation was insensitive to pyrazosulfuron-ethyl (resistance index (RI) = 310.0) and penoxsulam (RI = 10.0), whereas the enzyme from YX12--10 and YX15-38 was insensitive to pyrazosulfuron-ethyl, penoxsulam, imazapic and bispyribac‑sodium (RI values ranging from 4.3 to 4462.0). AHAS inhibitor cross-resistance bioassays showed that YX12-10 and YX15-38 had cross-resistance to all of the tested herbicides (RI values ranging from 5.8 to 3321.9), while the YX15-22 population only had resistance to pyrazosulfuron-ethyl (RI = 827.4) and penoxsulam (RI = 6.6). This study clarified the distribution of resistant C. difformis in China and the different cross-resistance patterns given by various mutation types of AHAS.

Inheritance and Molecular Characterization of a Novel Mutated AHAS Gene Responsible for the Resistance of AHAS-Inhibiting Herbicides in Rapeseed (Brassica napus L.)

The use of herbicides is an effective and economic way to control weeds, but their availability for rapeseed is limited due to the shortage of herbicide-resistant cultivars in China. The single-point mutation in the acetohydroxyacidsynthase (AHAS) gene can lead to AHAS-inhibiting herbicide resistance. In this study, the inheritance and molecular characterization of the tribenuron-methyl (TBM)-resistant rapeseed (Brassica napus L.) mutant, K5, are performed. Results indicated that TBM-resistance of K5 was controlled by one dominant allele at a single nuclear gene locus. The novel substitution of cytosine with thymine at position 544 in BnAHAS1 was identified in K5, leading to the alteration of proline with serine at position 182 in BnAHAS1. The TBM-resistance of K5 was approximately 100 times that of its wild-type ZS9, and K5 also showed cross-resistance to bensufuron-methyl and monosulfuron-ester sodium. The BnAHAS1^544T transgenic Arabidopsis exhibited higher TBM-resistance than that of its wild-type, which confirmed that BnAHAS1^544T was responsible for the herbicide resistance of K5. Simultaneously, an allele-specific marker was developed to quickly distinguish the heterozygous and homozygous mutated alleles BnAHAS1^544T. In addition, a method for the fast screening of TBM-resistant plants at the cotyledon stage was developed. Our research identified and molecularly characterized one novel mutative AHAS allele in B. napus and laid a foundation for developing herbicide-resistant rapeseed cultivars.